Measuring permeability by the thermal expansion method for rigid or highly permeable gels

The permeability (D) of a gel can be determined by analysis of its thermal expansion kinetics: as the gel is heated, the expanding liquid stretches the solid network like a spring; during an isothermal hold, the liquid drains out and the gel returns to its initial dimensions at a rate that depends on D and the elastic modulus of the network. However, if the network is too rigid or D is too high, the dilatation of the network may be too small to measure easily. The measured expansion increases with the viscosity of the pore liquid, so D is easier to measure after the gel is rinsed in a higher alcohol. For example, a thermally aged silica gel that shows no measurable expansion in ethanol exhibits a large thermal strain after the pore liquid is changed to 1-octanol. It is important to demonstrate that the same permeability is obtained regardless of the liquid employed (i.e., that flow in the small pores of the gel does not give rise to liquid-specific effects), so we examine the thermal expansion of a silica gel rinsed successively in ethanol, 1-octanol, and 1-decanol; the gel is compliant enough so that D is measurable in all these liquids. The permeability is found to be the same, but the expansion is much more easily measured for the more viscous alcohols.